We plan to study the mechanism of B12 absorption by: a) isolating and purifying the intrinsic factor (IF) receptor protein from the ileal intestinal epithelium of the guinea pig, b) determining the mechanism by which the IF-B12 enters the enterocyte and localizes to the mitochondrion, c) determining the part of the mitochondrion the IF-B12 complex localizes, d) studying the enzymic mechanism which dissociates the intracellular IF-B12 complex, and e) determining the nature of other forms of bound B12 in the ileal enterocyte. We plan to develop radioimmunoassays which are specific for each form of B12 binding protein and to apply these assays to measure the relative concentrations of these binders in serum, body fluids, and tissues in health and disease. We plan to study methods of coupling different forms of B12 to immunogenic macromolecules in order to obtain antibodies with binding determinants specific for each analogue. If successful we will try to develop radioimmunoassays for each analogue, and also use labeled antibody to determine which form, and where each analogue localizes in cells of various tissues.